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Marketable Securities - Gains and Losses (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Gain (Loss) on Investments [Line Items]    
Amortized cost $ 26,435 $ 26,096
Gross unrealized holding gains 68 117
Gross unrealized holding losses (138) (57)
Estimated fair value 26,365 26,156
Less than 12 months, Estimated fair value 11,930  
Less than 12 months, Gross unrealized holding losses (92)  
More than 12 months, Estimated fair value 3,938  
More than 12 months, Gross unrealized holding losses (46)  
Total, Estimated fair value 15,868  
Total, Gross unrealized holding losses (138)  
Corporate bonds and notes    
Gain (Loss) on Investments [Line Items]    
Amortized cost 20,827 19,804
Gross unrealized holding gains 50 89
Gross unrealized holding losses (133) (55)
Estimated fair value 20,744 19,838
Less than 12 months, Estimated fair value 10,256  
Less than 12 months, Gross unrealized holding losses (88)  
More than 12 months, Estimated fair value 3,298  
More than 12 months, Gross unrealized holding losses (45)  
Total, Estimated fair value 13,554  
Total, Gross unrealized holding losses (133)  
Municipal bonds    
Gain (Loss) on Investments [Line Items]    
Amortized cost 5,608 6,292
Gross unrealized holding gains 18 28
Gross unrealized holding losses (5) (2)
Estimated fair value 5,621 $ 6,318
Less than 12 months, Estimated fair value 1,674  
Less than 12 months, Gross unrealized holding losses (4)  
More than 12 months, Estimated fair value 640  
More than 12 months, Gross unrealized holding losses (1)  
Total, Estimated fair value 2,314  
Total, Gross unrealized holding losses $ (5)